The raw material for cement clinker manufacturing can contain chromium. Due to the highly oxidizing and alkaline conditions of the kiln, during clinker production it is partially converted to toxic hexavalent chromium. As a result, the Portland clinkers and cements obtained by clinker grinding contain soluble chromates (usually in the range of 1-100 ppm or mg/kg, while the total chromium can reach the 500 ppm) that are released when cement comes into contact with water and are reported to cause skin irritation (allergic contact dermatitis). This is the reason why the European Community has recently introduced the obligation (Directive 2003/53/EC) to maintain the level of soluble chromates below 2 ppm (mg/kg) [see for example http://eur-lex.europa.eu/LexUriServ/site/en/oj/2003/I—178/I—17820030717en00240027.pdf].
Several reducing agents have been disclosed: see for example WO 2006061127, EP 1559694, EP 1533287, WO 2005090258, US 2005072339, WO 2005076858, EP 1505045, EP 960865, EP 697380, DE 10014468, FR 2858612, AU 647118, JP 2002274907, JP 11092193, CN 1299788.
The elimination of soluble hexavalent chromium is currently mainly obtained with the use of Iron(II) sulphate or Tin(II) sulphate or chloride (either in powder or in form of liquid additive) added during cement grinding or storage. Cr(VI) is electrochemically reduced to Cr(III) that is less toxic and anyway tends to precipitate in the alkaline conditions commonly found during the hydration of cement.
The use of Iron(II) salts is however not efficient, due to the fact that they are readily oxidized to Iron(III) after contact with air and moisture, particularly at high temperature (above 60° C.). The required amount of Iron(II) needed for the reduction of hexavalent chromium is then usually several times higher than stoichiometric, and after a prolonged contact with air (e.g. during storage) the reducing effect is lost. For the same reason, the use of Iron(II) compounds in water solution is not useful. Iron(III) produced from oxidation of Iron(II) is also reported to cause spots on the surface of cement and Iron(II) sulphate at high dosages can cause an extended setting time in cementitious systems.
The use of stannous salts allows to obtain better performances: the dosages are close to stoichiometric and the reducing effect is higher and longer-lasting in comparison to Iron(II). The oxidation products of Tin(II) do not cause spots on concrete surface. The reducing properties of Tin(II) in alkaline conditions are based on the formation of stannous hydroxide Sn(OH)2 that has a red-ox potential E=−0.96 volt at pH=13 [Handbook of Chemistry and Physics, CRC Press, 83rd edition, 2002-2003]. As soon as the cement is mixed with water, the pH rises and Tin(II) becomes a strong reducing agent that immediately eliminates soluble chromates.
If the formation of stannous hydroxide occurs when the hexavalent chromium is not yet available for reduction (e.g. during grinding or storage of cement or in general before mixing cement with water), Tin(II) is oxidized by air due to his low red-ox potential. Even in absence of air, stannous hydroxide tends to dismutate to Tin(0) and Tin(IV), being deactivated [A. Aràneo, Chimica analitica qualitativa, CEA, 3rd edition]. Stannous salts have acid properties and in presence of traces of water they can react with the very alkaline lime giving stannous hydroxide, according to the following reactions:SnSO4+CaO+H2O═Sn(OH)2+CaSO4  1SnCl2+CaO+H2O═Sn(OH)2+CaCl2  2
This is the reason why the stannous salts are not profitable when they are used in grinding of clinker with high amount of free lime: stannous hydroxide can be formed and quickly oxidized to Tin(IV) and the amount of Tin(II) must be increased in order to obtain the elimination of soluble chromates [see for details: “Stannous sulphate: research study” in World Cement, February 2007 issue].
The tendency to react with CaO in presence of water should be higher for stannous sulphate, thanks to the formation of the poor soluble calcium sulphate that shifts the reaction 1. This is probably the reason why stannous sulphate is less effective in solution (that means presence of water) than stannous chloride, as reported in 2005/0166801 A1 and in the example of the present patent. In the case of a clinker with a high content of free lime the results obtained either with stannous chloride or sulphate are not completely satisfactory (see Table). In particular the Tin(II) sulphate solution is less efficient than Tin(II) chloride solution.
In JP2003292351, EP1439154A1 and JP2004051424 a cement admixture capable of eliminating hexavalent chromium is disclosed. Said cement admixture basically comprises a slowly cooled blast furnace slag and the elimination of hexavalent chromium is obtained by electrochemical reduction (the reducing agent is a sulphur compound in non-sulfuric or non-sulphate form) or by suppressing the elution of soluble chromates (using conventionally known hazardous heavy metals immobilizing agents such as bentonites, zeolites or antimonates). Even though said admixtures is effective for use in concrete, the requirement of a high content of slowly cooled slag and the high dosages proposed (most preferably from 10 to 40 parts, in 100 parts of cement composition) lead to the disadvantage that it cannot be used for hexavalent chromium reduction in common cements. As a matter of fact, according to well known international standards (see for example the European Standard UNI EN 197-1: “Composition, specifications and conformity criteria for common cements”) common cements cannot contain such type of slowly cooled slag in the amount proposed in the cited patents. In particular, the characteristics of such slowly cooled blast furnace slag do not comply with the requirement for blast furnace slag commonly used in cements, as described in the EN 197-1.
In view of the disadvantages of the known reducing or immobilizing agents, there exists a need for a novel hexavalent chromium reducer, which can be used in cement grinding especially (but not limited to) in the case of clinker with a high content of free lime.